Onboard Tillman inspired JFET preamp

Discussion in 'Pickups & Electronics [BG]' started by MikeTranch, Feb 21, 2019.


  1. MikeTranch

    MikeTranch

    Aug 30, 2016
    I've built a few of the "Tillman" style preamps, into DIY pedals, cables, and into guitars. About a year ago I installed one in my Pbass. I've been playing it over a year and finally getting around to posting it here.

    About the switch: I used a sub-mini toggle on the pickguard, mounted between the tone control and output jack, for active/passive switch, and it's powered by +9V via TRS jack, and TRS cable from my pedal board. Or I can use a standard (TS) cable, for passive only, in which case the switch acts as a mute or kill switch. The switch is silent, both electrically and mechanically. In the schematic below, you'll see the switch is only SPDT, merely selects the passive signal or the buffered signal. In other words the JFET gate is always connected, but keep in mind, that load impedance is on the order of 10M and very low capacitance too. Key point: not switching power to the JFET eliminates pops when changing the switch.

    When active, the volume boost is moderate, I'd guess about 2 to 3 dB. One reason for this mod was to get a hotter signal for an envelope filter pedal that wasn't fully triggering. Since then I've ditched that pedal but I use this preamp, on and off, all the time. Note the volume control is before the JFET. At about 7 and above on the volume control, the boost transitions into mild compression. Being a FET, this is a nice sound, IMO, similar to a tube.

    Note the drain and source resistor values need to be adjusted for the particular J201B. But once you get it dialed in, it's stable. I've been using a few of these style preamps in my rigs for several years and they're all still working great.

    Here's the schematic. I had previously modified the tone control but that's obviously independent of the boost mod.

    Pbass FET zoomed.png
     
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  2. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Cool, I've built a number of variants myself. Have you found a reliable vendor for genuine J201s?
     
  3. MikeTranch

    MikeTranch

    Aug 30, 2016
    if I remember right, I got them on Amazon, five for $10 or so? a few years back. They all had reasonable gain. I've also used this circuit on guitars, and it can be a bit noisy (white noise) if going into a high gain chain. But in the case, the passive mode is fine...

    p.s. I also have active bass/guitars, designed with 9V batteries, that I routinely power the same way. I put a small fuse say, 0.2A, across the battery clip... back-drive the TRS jack... I'm considering a post about that... will search forum first... probably already covered...
     
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  4. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Nice. There have definitely been some other threads about that, but the more the better as far as I'm concerned. I'm working up a bass with +/- 15V on a 4 pin XLR at the moment, but in the end using a TRS and keeping things simpler could happen. That's one nice thing about using a standardized D size panel connector. Yel_wink.gif
     
  5. BassLife77

    BassLife77

    Nov 13, 2009
    San Diego
    I've built several of those J201 preamps and they sound better when the drain is half the supply voltage (4.5v) I substitute the 2K2 with a 5K trimpot
     
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  6. MikeTranch

    MikeTranch

    Aug 30, 2016
    Good tip Basslife77. That's how I breadboarded it, tweaked the pot to set the drain to about 4.5V, then replaced with a fixed value resistor. Lots more info on biasing at the link above. I did the same thing for the drain resistor value, found the value to get a full swing output with the volume at 10, giving a nice warm compression IMO. These JFETs have a wide range of gain and Vgs, so you pretty much have to do this for each one.

    p.s. I usually run my volume at about 7, gives a nice thump. The low output impedance is great for driving any length cable and stomp box you want.
     
    SteveCS likes this.
  7. Great thread.

    I built up two Tillman preamps into a box with seperate ins and outs but jack-switched to be TRS/TS in and two outs and mini switched to combine the outs for signal blending.

    It was for Chapman Stick and has been one of my most useful tools.

    The outboard pickup power has been on my mind for some years but never got round to it. Could be time...
     
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  8. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Some different spins on the very basic "Tillman" format, which is just a basic cookbook circuit really: FET Preamplifier, Part 1
     
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  9. I'm surprised. I've also built many fet preamp designs over the years. I'd expect far more gain than this from a J201 common source amp.

    The problem with a simple circuit like this is that the bias point won't stay at 1/2 Vcc as the battery voltage dies. It will drop faster than the battery voltage. I would normally set Vd up at 5.5-6V. This way, you'll still have usable headroom with the battery down on 6-6.5V.
     
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  10. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    It looks like InterFET has taken over the Vishay/Siliconix jFET business. You can get them at Mouser.

    InterFET Corp. Page Selector - J201 Datasheet
     
    Last edited: Feb 23, 2019
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  11. BassLife77

    BassLife77

    Nov 13, 2009
    San Diego
    When the battery voltage drops to 8 I always dump it. I turned the pot while playing and it sounded the best at 4.5v
     
  12. MikeTranch

    MikeTranch

    Aug 30, 2016
    UPDATE ON GAIN (5/2/2019)

    I finally got around to measuring the gain, and it's about 12 dB. That's comparing active mode to passive mode, with volume and tone full up, and playing one open string at a time, hard enough to almost slap the frets.

    Note the volume and tone are before the preamp. So in active mode, the volume control is like a "drive" control for the preamp.

    I usually play with tone rolled down and volume about 6 or 7, as I like that sound/tone, in both passive and active mode. With those settings, I'd say the volume doesn't quite double, so based on that, I had estimated 2 to 3 dB.


     
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  13. axelrod

    axelrod Supporting Member

    Jun 11, 2013
    ohio
    Built this buffer booster and now trying to figure out how to run it passive both on or just buffer. the input is coming from the passive tone output. This configuration works but passive volume drops precipitously and I think the booster and buffer are both operating together in all but passive.
    boostbusfswch.jpg
    Input to center left-output center right
    boost buff input bottom left-boostbuff out bottom right
    boost out top left-bridged-buffer in top right.
    design from tonefiend-seymore duncan
     
  14. axelrod

    axelrod Supporting Member

    Jun 11, 2013
    ohio
    Doesn't work but noise comes out. on-on-on
     
  15. rutrho

    rutrho

    Mar 29, 2014
    San Jose, CA
    Oh this is kind of interesting. Going to go dump it into my file of stuff to breadboard as I explore more onboard pre-amp options.
     
  16. axelrod

    axelrod Supporting Member

    Jun 11, 2013
    ohio
    Here are some other switching options I've tried.
    boostbusfswch2.jpg
     
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  17. axelrod

    axelrod Supporting Member

    Jun 11, 2013
    ohio
    Here’s the insides
    7AEAD118-D1A0-4DBE-A20D-40AA1BBDC726.jpeg 7AEAD118-D1A0-4DBE-A20D-40AA1BBDC726.jpeg
     
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  18. axelrod

    axelrod Supporting Member

    Jun 11, 2013
    ohio
    And the outside
    IMG_0011.JPG Still interested in switching ideas. Currently have just straight up active passive on off on.
     
  19. denverbarnes

    denverbarnes

    Feb 8, 2017
    Wow what is that body wood?
     

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